How to get Cycles viewport output in compositing
Well there is a way but it really is a cart before the horse thing. If you really need the image you can only get it by a screenshot, which will be limited to an 8 bit depth and the size of your viewport, obviously.

When it is saved you can then use a file node to import it into the node editor.
As I said this is a nuts way to do it but it can be done.

Don't do it though use F12.